Apparatuses for amplitude shift keying in the state of the art involve a class E amplifier including a switching transistor, whose gate is operated at an operating frequency, and at least one variable resistor, which is switched as a function of a simulating input signal, in order to effect the amplitude modulation of the output signal of the class E amplifier.
Such an apparatus is disclosed, for example, in the published European application EP 1480156A2, wherein, in such case, a voltage having the operating frequency is supplied via a so-called tri state circuit to the gate of the switching transistor. The tri state circuit is a network of variable resistances, which sums as a function of the input signal voltages U1-U4 and supplies to the gate of the switching transistor. Corresponding to the variable gate voltage of the switching transistor, the output signal of the class E amplifier has differently large amplitude. Therewith, the described apparatus is suitable for coding by means of amplitude shifting, wherein the tri state circuit and its operation mean a certain amount of complexity, which should be avoided.
The company, Endress+Hauser, manufactures measuring devices under the mark MEMOSENS, in the case of which a sensor module is combined with an electronics module, which is fixedly connected with the sensor module and arranged in a plug head housing. The plug head housing forms the secondary side of an inductively coupling, plug connector coupling for transmission of data and energy. The primary side element of the plug connector coupling, which is connected via a cable via a measurement transmitter, contains an apparatus of the state of the art, wherein the amplitude shift keying occurs via the at least one variable resistor, which is implemented as a series connected, switchable load in the class E amplifier.
Although the described procedure basically functions and has proved itself in measurement operation, nevertheless the following improvement opportunities can be named:
First, for modulation, a load current is required, which is associated with efficiency losses;
Second, the degree of modulation is limited by the load resistance;
Third, overshooting of edges can occur in the circuit, caused by parasitic capacitances; and
Fourth, extra components are required for the modulation, namely a switching transistor, a diode and three resistances.